Raman spectroscopy stems from the inelastic scattering of light by molecular vibrational energy levels. Raman spectroscopy as an analytical tool has been known for decades, and is particularly popular for several reasons. For example, molecular composition can be determined in the presence of water. Visible light can be employed for analysis allowing for the use of conventional fiber optics. Unique spectral fingerprints allow for identification and quantification of a wide variety of solids, liquids, and gases. One of the significant disadvantages of Raman spectroscopy is the inadequate sensitivity for trace or ultratrace analysis. This stems from the inherently weak nature of Raman scattering.
In the early 1970s several researchers found an anomalous enhancement of Raman scattering at the surface of certain metals. It has subsequently been found that the metals that have both practicality and strong enhancing properties are silver, gold, and copper. The enhancement is believed to generally come from an electromagnetic effect and in some cases, an enhancement due to the nature of the chemical bond to the metal surface has also been found. The reported enhancement for SERS depends on the structure of the surface and ranges from about 105 to 108. This discovery immediately made it possible to detect very small amounts of material adsorbed to these surfaces. The SERS effect is limited to molecules attached to or in very close proximity with the surface.
The drawback to conventional SERS is that it is limited to analytes that will naturally adsorb to a SERS active metal surface. Thus, while in special cases SERS provides sensitive detection, in most cases it suffers from the inability of the molecule to adsorb to the surface and to benefit from the SERS effect.
A method to overcome the lack of adsorptivity to SERS surfaces by an analyte, is to provide surface coatings that have an affinity for the analyte. An example is an early publication which describes using a surface bound coating in the detection of hydrogen ions at a surface using SERS (Determination of pH with SERS Fiber Optic Probes. Ken I. Mullen, DaoXin Wang, L. Gayle Hurley, and Keith Carron Anal. Chem., 64, 930, 1992). This publication showed that it was possible to permanently attach a coating to a SERS surface and to have the coating provide the affinity for the analyte.
More recently, it has been demonstrated that an irreversible covalent bonding reagent could be used to achieve even more sensitive detection. Furthermore, it was shown that the surface need not be coated with the surface bound reagent, but rather, the reagent could have two reactive sites. One site is analyte specific and the other is surface binding specific. This produces a high affinity permanent bond to the analyte and a high affinity permanent bond to the surface. An example of a dual binding reagent for trace detection is a reagent that binds bilirubin and which has an argentiphillic sulfide group to bind to silver (Surface Enhanced Raman Assays (SERA): Measurement of Bilirubin and Salicylate, Roberta Sulk, Collin Chan, Jason Guicheteau, Cieline Gomez, J. B. B. Heyns, Robert Corcoran, and Keith Canon, J. Raman Spectrosc., 1999, 30, 853-859).
Accordingly, an assay is needed which is capable of producing a SERS active solution that is sensitive to a specific analyte or group of analytes.